News Release Number: STScI-2007-01

Hubble Maps the Cosmic Web of "Clumpy" Dark Matter in 3-D

The full news release story:

An international team of astronomers using NASA's Hubble Space Telescope has created a
three-dimensional map that provides the first direct look at the large-scale distribution of dark
matter in the universe.

Dark matter is an invisible form of matter that accounts for most of the universe's mass.

The map provides the best evidence yet that normal matter, largely in the form of galaxies,
accumulates along the densest concentrations of dark matter. The map reveals a loose
network of filaments that grew over time and intersect in massive structures at the locations
of clusters of galaxies.

The map stretches halfway back to the beginning of the universe and shows how
dark matter has grown increasingly "clumpy" as it collapses under gravity.

This milestone takes astronomers from inference to direct observation of dark matter's
influence in the universe. Previous studies of dark matter are based largely on numerical
simulations of the expected evolution of large-scale structure. This evolution is driven by the
gravitational attraction of dark matter.

Mapping dark matter's distribution in space and time is fundamental to understanding how galaxies
grew and clustered over billions of years. Tracing the growth of clustering in the dark matter may
eventually also shed light on dark energy, a repulsive form of gravity that influences how dark matter
clumps.

The new maps of dark matter and galaxies will provide critical observational underpinnings to future
theories for how structure formed in the evolving universe under the relentless pull of gravity. Theories
suggest the universe transitioned from a smooth distribution of matter into a sponge-like structure of
long filaments.

The research results appeared online today in the journal Nature and were presented at the 209th
meeting of the American Astronomical Society in Seattle, Wash., by Richard Massey for the dark
matter and Nick Scoville for the galaxies. Both researchers are from the California Institute of
Technology, Pasadena, Calif.

"It's reassuring how well our map confirms the standard theories for structure formation." said Massey.
He calls dark matter the "scaffolding" inside of which stars and galaxies have been assembled over
billions of years.

Researchers created the map using Hubble's largest survey of the universe, the Cosmic Evolution
Survey ("COSMOS") with an international team of 70 astronomers led by Scoville. The COSMOS survey
covers a sufficiently wide area of sky – nine times the area of the Earth's Moon. This allows for the
large-scale filamentary structure of dark matter to be evident. To add 3-D distance information, the Hubble
observations were combined with multicolor data from powerful ground-based telescopes. "The 3-D
information is vital to studying the evolution of the structures over cosmic time," said Jason Rhodes, a
collaborator in the study at the Jet Propulsion Laboratory in Pasadena, Calif.

The dark matter map was constructed by measuring the shapes of half a million faraway galaxies. To reach us,
the galaxies' light has traveled through intervening dark matter. The dark matter deflected the light slightly
as it traveled through space. Researchers used the observed, subtle distortion of the galaxies' shapes to
reconstruct the distribution of intervening mass along Hubble's line of sight  a method called weak gravitational
lensing. This effect is analogous to deducing the rippling pattern in a glass shower door by measuring how light
from behind it is distorted as it passes through the glass.

"Although this technique has been employed previously, the depth of the COSMOS image and its superior
resolution enables a more precise and detailed map, covering a large enough area to see the extended
filamentary structures," said co-investigator Richard Ellis of the California Institute of Technology.

For astronomers, the challenge of mapping the universe has been similar to mapping a city from nighttime aerial
snapshots showing only streetlights. Dark matter is invisible, so only the luminous galaxies can be seen directly.
The new images are equivalent to seeing a city, its suburbs and country roads  in daylight, for the first time.
Major arteries and intersections become evident, and a variety of neighborhoods are revealed.

A separate COSMOS team led by Scoville presented images of the large scale galactic structures in the same
area with the dark matter. Galaxies appear in visible light seen with Hubble and in ground-based Subaru telescope
images by Yoshiaku Taniguchi and colleagues. The hot gas in the densest galaxy clusters was imaged in X-rays
by Gunther Hasinger and colleagues using the European Space Agency's XMM-Newton telescope.

Galaxy structures inside the dark matter scaffolding show clusters of galaxies in the process of assembly. These
structures can be traced over more than 80 million light-years in the COSMOS survey – approximately five times
the extent of the nearby Virgo galaxy cluster. In the densest early universe structures, many galaxies already have
old stellar populations, implying that these galaxies formed first and accumulated the greatest masses in a
bottom-up assembly process where smaller galaxies merge to make bigger galaxies  like tributaries converging to
form a large river.

The COSMOS survey shows that galaxies with on-going star formation, even to the present epoch, dwell in less
populated voids and dark matter filaments. "It is remarkable how the environment on the enormous cosmic scales
seen in the dark matter structures can influence the properties of individual stars and galaxies  both the maturity
of the stellar populations and the progressive 'downsizing' of star formation to smaller galaxies is clearly dependent
on the dark matter environment," said Scoville.

"The comparison is of fundamental importance," said Massey. "Almost all current scientific knowledge concerns only
baryonic matter. Now that we have begun to map out where dark matter is, the next challenge is to determine what it
is, and specifically its relationship to normal matter."

In making the COSMOS survey, Hubble photographed 575 slightly overlapping views of the universe using the
Advanced Camera for Surveys' (ACS) Wide Field Camera onboard Hubble. It took nearly 1,000 hours of observations.
Thousands of galaxies' spectra were obtained by using the European Southern Observatory's Very Large Telescope in
Chile, and the Subaru telescope in Hawaii. The distances to the galaxies were accurately determined through their
spectral redshifts. The distribution of the normal matter was partly determined with the European Space Agency's
XMM-Newton telescope.